The issue: when worlds that were previously loaded in forge are loaded in vanilla, all observers are frozen in the powered state and are unresponsive to input. Even if the world is then reloaded in Forge, the observers are still frozen.
I did do some research on this issue beforehand, and according to this thread
it was fixed back in August. However, it appears to have resurfaced. This is a huge problem for me, as some of my worlds I use in both modded and vanilla.